This invention relates to a method of lubricating a journal bearing system of an industrial machine, or a high-precision machine tool such as a grinding machine, a lathe and a machining center. This invention also relates to a method of temperature control by using such a lubrication method.
As the speed of rotation of a main shaft of a machine tool increases, the temperature of its bearing rises accordingly faster and such a rise in temperature often dictates the maximum operating speed of the shaft. Since the main shaft and its support may undergo a thermal deformation which may adversely affect the work precision, it has been an important problem how to limit the rise in the temperature of the bearing. FIG. 1 shows a prior art device for supplying a lubricant to a bearing wherein a spindle unit 1 is shown as comprising a main spindle 2 having a tool attached to its tip, being rotatably supported by a bearing (such as a hydrodynamic bearing or a ball bearing), and being driven through a belt or by being directly connected to a motor 3. An oil pump 5 for the lubricant supplying device is connected to an oil tank 7 through a filter 6 and is adapted to discharge oil at a fixed rate by the operation of a pump-driving motor 8. The discharged lubricant oil is supplied to the spindle unit 1 through a flow rate adjusting valve 9, a pressure control valve 10 and a supply flow route 11. After lubricating the bearing, the oil is then returned to the oil tank 7 through a discharge flow route 12. The flow rate adjusting valve 9 is usually set appropriately in view of the flow required by the bearing. The portion of the oil not supplied to the bearing is returned to the oil tank 7 through a discharge route 13 of the pressure control valve 10.
The oil pump 5 and the pump-driving motor 8 are designed to have a larger-than-necessary operating capacity. Thus, the portion of the oil not necessary for lubrication is discharged through the discharge route 13 of the pressure control valve 10 and thereby raises the temperature of the oil inside the tank 7. This means that the oil pump 5 and the pump-driving motor 8 are wasting energy to the extent represented by the discharged amount of the oil. This wasted energy ends up as a rise in the temperature of the lubricant supplying device as a whole and also that of the lubricant oil in addition to the regular rise in the temperature of the oil due to the generation of heat accompanied by the rotary motion of the main shaft inside the bearing unit. In other words, the overall temperature of the bearing unit approaches more quickly the maximum allowable level for the operation of the unit, while the thermal deformation of the machine is increased.
Although an attempt may be made to adjust the flow rate of the lubricant oil by means of a flow rate control valve, the flow rate is not optimized in most situations and the adjustment is usually made on the larger side. This results in a random flow pattern inside the bearing unit and tends to raise the temperature.
It is therefore an object of this invention in view of the above to provide an improved method of lubricating a bearing of a rotary shaft, instead of the prior art mechanism for supplying a bearing unit a lubricant which has been heated additionally due to the stirring inside the bearing unit and the extra energy generated by the lubricant supplying device itself.
It is another object of this invention to provide a method of controlling the temperature of a rotary shaft by using such an improved method of lubrication.
According to this invention, use is made of an oil pump adapted to supply a lubricant to the bearing at different flow rates depending upon how fast the oil pump is operated, say, by means of a motor. The oil pump is preliminarily operated such that the user can derive, or determine, a relationship between temperature rise in the bearing and the flow rate of the lubricant by the oil pump with the spindle speed as a parameter. If this relationship is drawn as a curve in a graph, the graph shows a minimum point, indicating that there exists an optimum flow rate at which the temperature rise of the bearing can be minimized. After such a relationship is established, the oil pump is operated such that the lubricant is discharged at the optimum flow rate thus determined according to the current rotary speed of the shaft.
From such curves corresponding to different rotary speeds of the shaft, a temperature characteristic curve representing these optimum flow rates of the lubricant can be obtained corresponding to different rotary speeds of the shaft, and this temperature characteristic curve may be used to vary the speed of operation of the oil pump to change the flow rate of the lubricant according to different rotary speeds of the shaft.
When it is desired to increase the temperature of the bearing faster such that it can reach its saturation point more quickly, the oil pump can be operated at a faster rate such that the flow rate of the lubricant will be much greater than the optimum rate determined as described above and then reduced after the saturation point is reached.